Device for inline measurement laser pulses and measuring method by photoacoustic spectroscopy

ABSTRACT

A piezoelectric sensor ( 7 ) is dedicated to making a precise measurement of the light amplitude of pulses ( 2 ) from a laser ( 1 ), so that quantitative measurements such as photo-acoustic spectroscopy can be used to precisely measure the concentration of some bodies in a solution.

The purpose of this invention is a device for online measurements oflaser pulses, applicable particularly to a measurement method byphoto-acoustic spectroscopy that forms another purpose of thisinvention.

Photo-acoustic spectroscopy consists of sending light pulses throughsome absorbent solutions and particularly lanthanides, actinides,fission products (FP) (containing ions such as Nd³⁺ or Pr³⁺) or uranium.The optical energy produces a state change of the electrons of dissolvedbodies in the case of a non-radiative de-excitation and emission of anacoustic wave in the solution, that can be measured by a sensor. Oneadvantage of this spectroscopy method is that it then detects very lowconcentrations of dissolved bodies with this energy conversion property;but although it can easily be applied to qualitative measurements,before the invention it was impossible to precisely deduce theconcentration of the body in the solution in which the acoustic waveproduced was measured; the energy of the acoustic wave is proportionalto the optical energy at the same time as the concentration of the body,so that both of these values have to be known. Pyroelectric orphotoelectric probes were used in the past to estimate the energy oflaser light pulses, but the probe had to be subjected to a series ofpulses to obtain a measurable result, from which the energy of a pulsewas determined using a mean calculation; however, the variations in theamplitudes of the pulses are fairly sensitive.

Another idea would consist of using photodiodes to measure the amplitudeof each pulse, but these photodiodes have a transfer function thatvaries with the wavelength, whereas in general the solution needs to bescanned at different wavelengths. Finally, it should be emphasized thatonly pulsed light waves induce an acoustic wave proportional to theconcentration of the solution. Thus, the measurement of the energy oflaser pulses during each laser firing enables a precise normalization ofthe photo-acoustic signal generated in the solution for application ofthe formula:$\frac{A(\lambda)}{E(\lambda)} = {K\quad{ɛ(\lambda)}\quad C\quad{where}}$$\left\{ \begin{matrix}{A(\lambda)} & = & \text{Amplitude~~of~~the~~signal~~in~~the~~solution,} \\{E(\lambda)} & = & \text{Incident~~light~~energy,} \\{ɛ(\lambda)} & = & \text{Molar~~extinguishing~~coefficient~~in~~the} \\\quad & \quad & \text{absorbent~~species~~in~~solution,} \\C & = & \text{Molar~~concentration~~of~~the~~species~~in} \\\quad & \quad & \text{solution.}\end{matrix} \right.$

Thus, a first aspect of the invention is a device for on linemeasurement of laser pulse energies, that enables an independentmeasurement of the amplitude of each pulse effectively sent, and whichis not sensitive to the wavelength of light; this device consists of apiezoelectric sensor not provided with a prior protection layer and witha resonant frequency at least fifty times lower than the inverse of theduration of the pulses. The pressure wave produced in the piezoelectricsensor is then proportional to the amplitude of the incident lightpulse, without depending on its wavelength. The inventors observed thatprotection layers currently used in front of piezoelectric sensors inother applications to protect them from the direct impact of a wave,also dampened and clipped the laser pulse, which created an incorrectresponse at the sensor. They put forward the theory that this is theresult of multiple reflections of portions of the pulse at surfaces ofthe protection layer. These reflections in the protection layer absorban unknown quantity of the energy of laser pulses, and which depends onits wavelength; therefore, it is essential that the material from whichthe piezoelectric sensor is made is homogenous so that quantitativeresponses correlated to known incident energy can be measured, and tousefully carry out frequency scans, that are often indispensable inmolecular spectroscopy. Furthermore, elimination of the protection layeris a means of obtaining a single damped sinusoidal electrical responsesignal without any late echoes that would disturb synchronization of themeasurement. The sensor according to the invention is possibly coveredonly by a frequently used quarter wave blade in commercially availablesensors, which is why it is useful to adapt the acoustic impedancebetween the coupling liquid and the sensor while reducing acousticinterference, without the possibility of attenuations or measurablereflections occurring, due to its thinness. This sensor may be locatedon a portion of the laser beam that is diverted from a main path by aseparating blade; since the portion taken off is known and is constantover an extended but limited range of wavelengths, the measurement withthe piezoelectric sensor is a means of knowing the value of the rest ofthe pulse which is assigned to the test or the measurement andparticularly a photo-acoustic spectroscopy measurement as mentionedabove, that may include a light frequency scan.

The invention will now be described with reference to FIGS. 1 and 2 thatillustrate two possible embodiments of it. In FIG. 1, a pulsed laserperiodically emits a beam 2 towards a tank 3 containing a liquid sampleto be analyzed and the bottom of which is occupied by an acoustic sensor4 that measures the acoustic waves produced by transformation of theoptical beam 2 in the tank 3 in the presence of some dissolved bodies.However, the beam 2 passes through the separating blade 5 before itreaches the tank 3, which takes off the portion 6 that is diverted to apiezoelectric sensor 7 for which the measurements are used by apreamplifier 8 and by a measurement means 9. The signal from the sensor4 is amplified by a preamplifier 8′ similar to 8. The measurement means9 receives the two synchronized measurements through a signal receivedfrom laser 1, in each firing. The measurements output from detectors 4and 7 are compared, and the measurement means 9 uses the measures tonormalize the test result. The piezoelectric material of the detector 7is bare, in other words there is no layer supposed to protect it for thereasons mentioned above, but a frequently used quarter wave blade may beused without reducing the precision of the measurement. Tests have shownthat the current amplitude produced by sensor 7 by the piezoelectriceffect is proportional to the amplitude of the portion 6 of the pulse ofbeam 2, and the pulse itself. As shown in FIG. 2, an optical fiber 10may be placed on the path of the portion 6 between the blade 5 and thesensor 7, to guide the sampled portion of the pulses more easily.

We will now give a more detailed description of the apparatus; the laser1 may be a matchable pulsed laser with an optical parametric oscillatorfor light wavelength between 220 and 1800 nanometers, the pumping lasermay be a YAG, the firing frequency 10 hertz and the pulse duration 10nanoseconds. The length of the optical fiber 10, for example made ofsilica, may be 2 meters for a diameter of 550 micrometers, so as totransmit 99% of light on all wavelength ranges scanned in the visible.The piezoelectric sensor 7 may be cylindrical and its diameter may be 10millimeters, with a nominal frequency equal to 250 kilohertz, in otherwords with periods of 4 microseconds. More precisely, the ratio betweenthe pulse duration of laser 1 and the vibration period of thepiezoelectric sensor 7 must be at least fifty. This system was used toplot photo-acoustic spectra on solutions containing, for example,uranium IV lanthanides. Long series of tests were carried out atwavelengths of 450, 526, 650 and 800 nanometers of light and correlatedto measurements with calibrated pyroelectric probes; they demonstratedlinearity of the response of the piezoelectric sensor 7, and that it isindependent from the wavelengths. Total energies of pulses emitted tothe piezoelectric sensor 7 were 0.15 millijoules, and they producedamplitudes of a few hundred millivolts within a measurement range usingthe device 9 up to about 1 volt, so that the measurement should be validat energies up to the order of one millijoule.

It is expected that this method could be extended to includemeasurements of radioelements on nuclear fuel.

1. Device for online measurement of pulses (2) of a laser (1),characterized in that it comprises a piezoelectric sensor (7) notprovided with a protection layer and with a resonant frequency at leastfifty times lower than the inverse of the duration of the pulses. 2.Device for online measurement of pulses of a laser according to claim 1,characterized in that the sensor is located on a portion (6) of thelaser beam that is diverted from a main path by a separating blade (5).3. Device for online measurement of pulses of a laser according to claim2, characterized in that the piezoelectric sensor (7) and another sensor(4), that records a main measurement made using a portion of the beamcomplementary to the diverted portion, are connected to a singlemeasurement means (9).
 4. Photo-acoustic spectroscopy measurement, inwhich a medium (3) is irradiated by the pulses of a laser, characterizedin that the amplitude of each of the pulses is measured by the deviceaccording to claim
 3. 5. Photo-acoustic spectroscopy measurementaccording to claim 4, characterized in that radiation is made with lightfrequency scanning.